The present invention relates to steam turbines and, more particularly, to apparatus and method for controlling and testing steam control valves of steam turbines.
The shaft speed or torque of a steam turbine is controlled by a plurality of valves which meter steam into turbine wheels of the steam turbine. The principal control is accomplished by main control valves which control the entry of steam into first-stage nozzles of the high-pressure steam turbine. The first-stage nozzles are customarily divided into a plurality of subsets of nozzles, each supplying steam to an arcuate portion of the first-stage turbine wheel. Each of the subsets of nozzles is fed high-pressure steam from its own main control valve.
Some steam turbine applications such as, for example, commercial electric utility power generation, entail very long periods of operation without plant shutdown for periodic maintenance.
Thus, unsuspected valve failures may occur which can create serious problems when plant shutdown is required. For example, a valve stem may break and prevent closing of the valve. Alternatively, excessive friction may develop in the valve drive apparatus or the valve may become misaligned with its seat thus delaying valve closing or permitting only partial valve sealing respectively. Larger steam turbines employ separate valve actuators for each of the plurality of main control valves. It is thus possible to test closure of each of the main control valves in turn during periods when the full output capability of the steam turbine is not required. Some intermediate-sized steam turbines employ ganged control of a number of the main control valves. For example, a control system disclosed in U.S. Pat. No. 4,082,115, of common assignees with the present invention, employs a single actuator driving a plurality of cams which, in turn, control the positions of three or more valves. The cams are shaped to control steam entry to the first-stage turbine wheel in such a manner that substantially equal turbine output increases are produced for equal cam motion. It should be noted that a substantial non-linearity is built into the cam profiles to achieve such proportional control of turbine load. Valve actuation is sequenced by the cam action so that steam admission begins by opening one of the main control valves and progresses to opening the others in sequence until, at full load, all main control valves participate in supplying steam to the turbine. The shutdown sequence is essentially the inverse of the startup sequence. It would be clear to one skilled in the art that the first valve to be opened in such a cam-controlled valve system is also the last to be closed. Thus, if all of the main control valves are actuated by a single cam control system, only upon complete plant shutdown can closure of the first-opened main control valve be tested.
Some steam turbines separate cam-controlled main control valve actuation into two or more sets rather than a single set. One type of steam turbine, for example, employs two coordinated cam control devices for the concerted operation of six main control valves. Each of the cam control devices sequences the valve actuation for its set of three main control valves. In the startup sequence for the steam turbine, one of the cam control devices begins by opening one of its main control valves. This is followed by the second cam control device beginning opening one of its main control valves. The sequence continues with each cam control device in turn beginning the opening of one of its associated main control valves until all six main control valves are open for feeding steam to their associated subsets of nozzles. The shutdown sequence is essentially the reverse of the startup sequence described in the preceding with the cam control devices completing the closing of each main control valve in turn until the first-opened main control valve is at last closed. Thus, testing of closure of all main control valves can still only be performed in connection with a complete plant shutdown.